UTIs affect 7 million women each year and result in substantial morbidity, time lost from work and medical costs exceeding $1 billion per annum. Colonization of the vaginal introitis with E. coli is a critical first step in the pathogenesis of these infections. Preliminary data that we have collected suggest that vaginal colonization with hydrogen peroxide producing lactobacilli prevents both vaginal colonization with E. coli and UTI. The overall goal of this project is thus to delineate the protective role of H202 producing lactobacilli, the principal microbial species of the normal vaginal flora, in preventing E. coli vaginal colonization and UTI. We will accomplish the following specific aims: (1) In a prospective study of women followed for four months after treatment for UTI, we will ascertain the incidence of E. coli vaginal colonization and UTI in those with and without vaginal H202 producing lactobacilli. (2) In the same cohort, we will utilize daily diaries to identify specific exposures (spermicide, antimicrobials, sexual intercourse, new partner, douching, and others) that reduce the prevalence and quantity of vaginal H202 producing lactobacilli. (3) In a randomized trial, we will determine whether treatment of UTI with antimicrobials that reduce or eliminate vaginal H202 producing lactobacilli results in more frequent recurrences of UTI. (4) Using molecular characterization of the strains collected in the prospective study, we will determine whether specific urovirulence genes or the presence of interim vaginal H202 producing lactobacillus colonization affects recurrence rates. (5) We will perform a pilot study to determine whether a highly adherent H202 producing strain of Lactobacillus crispatus given as a vaginal suppository prevents vaginal colonization with E. coli. The proposed studies will established the importance of H202 producing lactobacilli in preventing UTI, and will identify factors adversely influencing the normal lactobacillus dominant flora. Development of an effective lactobacillus vaginal suppository offers a new approach to preventing UTIs, and could also have important implications for other infections.